The present invention relates to interface between an ISDN (Integrated Service Digital Network) subscriber switching unit and user terminal units in a digital communication system using an ISDN and, more particularly, to a processing system for saving a call between the ISDN subscriber switching unit and a given terminal unit, which call was present before the subscriber switching unit temporarily stopped its process, without disconnecting the call when the subscriber switching unit restarts the process, and a method therefore.
When a general subscriber switching unit stops (interrupts) a process during an operation, all calls in processing are lost/canceled. In contrast to this, in an ISDN switching unit, if information about such a call is left at the execution of a restart process, and continuation of a connection process or the like for the call is required by the terminal unit side, the call can be saved without being lost/canceled. If such a call saving process is properly executed, an excellent system for reducing the large load exerted on the control unit of the switching unit in a restart process can be realized. In addition, this call saving process is an important service which can be offered to the users.
A conventional digital terminal unit associated with this process is disclosed in detail in Japanese Patent Laid-Open No. 1-192246.
A call saving determination process based on a conventional technique will be described below with reference to FIG. 5. In this case, when a restart process accompanying a call saving operation is started, a subscriber switching unit 1 determines a call state managed by the subscriber switching unit 1. That is, the subscriber switching unit 1 determines a call other than a call under communication or call set up processing and the state of a resource associated with the call managed by the subscriber switching unit 1. If it is determined that the call cannot be saved, the call is determined as a save disable call. In this case, the subscriber switching unit 1 transmits a release complete message 11 to a terminal unit 3 for each call so as to perform a call disconnection/release process.
If it is determined that the call state managed by the subscriber switching unit 1 indicates a call under communication or call set up processing, and the state of the resource associated with the call managed by the subscriber switching unit 1 indicates that the call can be saved, the target call is determined as a save enable call. In this case, the subscriber switching unit 1 transmits a state inquiry message 12 to the terminal unit 3 for each call, and starts a timer (not shown). Upon reception of the state inquiry message 12, the subscriber terminal unit 3 sets a call state managed by the terminal unit 3 in a state display message 13, and transmits it to the subscriber switching unit 1.
Upon reception of the state display message 13 from the terminal unit 3, the subscriber switching unit 1 determines a state display information element in the state display message 13, thus determining whether the set state indicates a call under communication or call set up processing, and the call state managed by the subscriber switching unit 1 coincides with the state set in the state display information element. In addition, the subscriber switching unit 1 determines the state of the resource associated with the call managed by the subscriber switching unit 1. If there is no contradiction in the state of the resource, and the call can be saved, the target call is determined as a save enable call.
If the call state set in the state display information element in the state inquiry message 12 indicates a call other than a call under communication or call set up processing, or if the call state set in the state display information element indicates a call under communication or call set up processing, but the call state managed by the subscriber switching unit 1 does not coincide with the call state on the terminal unit 3 side which is set in the state display information element in the state display message 13, the target call is determined as a save disable call. In this case, the subscriber switching unit 1 transmits a release complete message 14 to the terminal unit 3 to perform a call disconnection/release process.
Assume that the state set in the state display information element in the state display message 13 indicates a call under communication or call set up processing, and the call state managed by the subscriber switching unit 1 coincides with the state set in the state display information element. In this case, if there is contradiction in the state of the resource associated with the call managed by the subscriber switching unit 1, and a call saving process cannot be performed, the target call is determined as a save disable call. In this case, the subscriber switching unit 1 transmits the release complete message 14 to the terminal unit 3 to perform a call disconnection/release process.
When a predetermined period of time set in the timer (not shown) of the subscriber switching unit 1 has elapsed, the target call is determined as a save disable call. In this case, the subscriber switching unit 1 transmits the release complete message 14 to the terminal unit 3 to perform a call disconnection/release process.
In such a conventional call saving signaling system, the subscriber switching unit 1, which starts a restart process accompanying a call saving operation, determines the state of a resource and a call state managed on the subscriber switching unit 1 side. The subscriber switching unit 1 then transmits the state inquiry message 12 for each of all calls determined as calls which can be saved. In addition, the subscriber switching unit 1 transmits the release complete message 11 for each of all calls determined as calls which cannot be saved, thus performing a call disconnection/release process. For this reason, the number of messages to be transmitted at the start of a restart process becomes large. Consequently, the signal processing load on the subscriber switching unit 1 is increased, and it takes much time to perform a restart process.